This invention relates to an art for converting progressive scan signals and, more particularly, to the art for sub-sampling chrominance signals of the progressive scan signal of 4:2:2P format (4:luminance, 2:chrominance or previously called 8:4:4, 8:luminance signal, 4:chrominance signal) for conversion into chrominance signals of the progressive scan signal of 4:2:0P format (or previously called 8:2:2, 8:luminance signal, 2:chrominance signal; it is also called as 4:2:2:4 format).
Aiming for satisfying needs for improving picture quality, various types of standards on high picture quality TV have been proposed. Among those standards, Enhanced TV in Japan (referred to as EDTV-II, hereinafter) has been highly expected as a target for practical implementation. For this, diversified developments have been on the progress.
Two formats of progressive scan signals have been proposed for the 525-line progressive scan signal. One is the progressive scan signal of 4:2:2P format for sub-sampling the luminance signal at 27 MHz and the chrominance signal at 13.5 MHz. The other is the progressive scan signal of 4:2:0P format for sub-sampling chrominance signals of 4:2:2P format vertically by half at a half sub-sampling rate, i.e., 6.75 MHz. The aforementioned progressive scan signals of 4:2:0P format is also called as 4:2:2:4 format, which has been disclosed in Japanese Patent Laid Open 46627 (1995) in detail.
At present a studio has generally employed two types of video equipment suitable for the use in either the 4:2:2P format or the 4:2:0P format.
In order to use those differently formatted units efficiently, a conversion technique between 4:2:2P format and 4:2:0P format has to be established.
In order to convert the 4:2:2P format into the 4:2:0P format, chrominance signals of the 4:2:2P format progressive scan signal are vertically sub-sampled by half. However vertical band-limitation prior to the sub-sampling is necessary to cope with aliasing caused by the vertical sub-sampling. The vertical band-limitation is done by low pass filtering the chrominance signals of the 4:2:2P format progressive scan signal in vertical direction. Contrary to this, conversion of the 4:2:0P format into the 4:2:2P format requires interpolation of the chrominance signal.
In the studio, the system is likely to repeat the conversion from the 4:2:2P format to the 4:2:0P format or vice versa several times.
In case of converting the 4:2:2P format into the 4:2:0P format, the vertical band-limitation should be executed only once before sub-sampling. From the second conversion onward, sub-sampling is only required because limiting the bandwidth at every sub-sampling may result in the signal attenuation, leading to troubles.
In order to solve the aforementioned problem, it has been proposed that an operator makes sure at every conversion if the signal is a band-limited signal or not. If it is confirmed that the signal has been bandwidth limited, the signal will be sub-sampled without filtering. This method is rather impractical because it is impossible to distinguish the exact signal from many other video signals processed in the same studio.
Another method has been proposed to insert identification information indicative of band-limitation into the signal. Detecting this identification information identifies the signal for sub-sampling without filtering.
This method calls for a certain standard specifying the position where such identification information should locate within the signal, requiring much time and labor.